The present invention relates to methods and apparatus for depositing epitaxial layers on wafers and other substrates, and more particularly, to improving the doping uniformity and related qualities of such layers.
The semiconductor industry employs crystal growth from vapor, in particular, for producing epitaxial layers on semiconductor wafers. The term epitaxy typically describes the growth of a monocrystalline layer on the planar boundary surface of a monocrystalline substrate, generally a substrate wafer of a semiconductor material.
Epitaxial growth is often carried out using chemical vapor deposition (CVD) in CVD reactors. In such processes, the semiconductor wafer is first heated and then exposed to a gas mixture, referred to as a process gas. The process gas mixture typically consists of a reactant gas, a carrier gas, and, where appropriate, a dopant gas. The reactant gas (or gases) provides the elements that form the desired epitaxial layer; e.g. silicon from silane and carbon from propane to form silicon carbide. The dopant gases carry elements, typically as compounds, that add p or n-type conductivity to the epitaxial layer; e.g. nitrogen to obtain n-type silicon carbide. The reactant and dopant gases react on or near the hot substrate surface to form the desired epitaxial layer.
In a typical CVD process, reactant gases at room temperature enter the reaction chamber. The gas mixture is heated as it approaches the deposition surface, for example, by radiative heating or by coming into contact with a heated surface. Depending on the process and operating conditions, the reactant gases may undergo homogeneous chemical reactions in the vapor phase before striking the surface. Near the surface, thermal, momentum, and chemical concentration boundary layers form as the gas stream heats, slows down due to viscous drag, and the chemical composition changes. Heterogeneous reactions of the reactant gases or reactive intermediate species (formed from homogeneous pyrolysis) occur at the deposition surface forming the deposited material. Gaseous reaction by-products are then transported out of the reaction chamber.
Commonly owned U.S. Pat. No. 6,569,250, incorporated entirely herein by reference, discloses a gas driven rotation apparatus for chemical vapor deposition of epitaxial layers on a semiconductor substrate. The previously described gas mixture enters the rotation apparatus from a location situated above the base portion. As the reactant gas mixture approaches the substrate surface, the reactant gases (after homogeneous chemical reaction, as previously described) are deposited on the substrate. Without being bound by theory, it is believed that the phenomenon of high dopant incorporation at the edge of wafers during standard epitaxial deposition is a result of a temperature gradient that exists across the wafer surface. Particularly, the edge of the wafer is almost always cooler than the center of the wafer, due to the greater surface area of the edge region. This greater surface area provides more directions for heat radiation than the center of the wafer.
A higher silicon to carbon ratio in the process gas may result in higher nitrogen (donor) dopant incorporation efficiency during silicon carbide growth, while a lower silicon to carbon ratio in the process gas may result in lower nitrogen dopant incorporation efficiency. The cooler edge regions of semiconductor wafers result in a higher effective silicon to carbon ratio on the wafer edge than the effective silicon to carbon ratio in the center of the wafer surface. Accordingly, the nitrogen dopant incorporation occurring at the center of the wafer may be lower that the nitrogen dopant incorporation at the wafer edge.
Doping changes in epilayers deposited on substrate wafers often lead to different performance among devices, such as Field Effect Transistors, that may be later fabricated on the wafer. Stated differently, because the doping levels in epilayers and wafers often exert some control over device performance, where multiple devices are fabricated on a single wafer, those devices will perform differently if the dopant levels are not substantially equal across the wafer surface. It would therefore be desirable to develop a method of depositing epitaxial layers substantially free of changes in the doping profile across the wafer surface.